Every year when school starts, you hear it in the classroom: a cough here, a snuffle there. Some weeks, more than half your class may be sneezing or hacking away. Colds spread quickly, passing from person to person. Then there's the flu season: sore throats, runny noses, fevers, aches and pains, and absences from school.

It could be worse. Earlier this year, many people died in China and other countries from a disease called SARS (Severe Acute Respiratory Syndrome). Some schools ...